1. Technical Field
Illustrative embodiments relate to a method of selecting a cell to be used for connection between a relay station and a base station from a plurality of candidate cells in a mobile communication system including a base station and a relay station that belongs to the base station.
2. Background
In LTE-Advanced (Long Term Evolution Advanced) of 3GPP (3rd Generation Partnership Project), the introduction of relay stations (hereinafter “RNs (Relay Nodes)”) has been examined. Relay stations of LTE-Advanced are shown in 3GPP TR 36.912 V9.2.0 (2010-03), “Feasibility study for Further Advancements for E-UTRA (LTE-Advanced)”, and 3GPP TR 36.806 V9.0.0 (2010-03), “Relay architectures for E-UTRA (LTE-Advanced)”. The RN is one of techniques for increasing the communication speed of mobile stations (hereinafter “UEs (User Equipments)”) located at cell edges and/or for increasing cell areas of base stations (hereinafter “eNBs (Evolved Node Bs)”). The details of the RN architecture examined in 3GPP are shown in 3GPP TR 36.806 V9.0.0 (2010-03).
The outline of a mobile communication system based on the RN architecture disclosed in 3GPP TR 36.806 V9.0.0 (2010-03) is explained hereinafter. FIG. 1 shows a network configuration example when the RN examined in 3GPP is used. A base station (eNB) 91 belongs to a core network (hereinafter “CN”) 4 of a mobile telecommunications carrier. The base station (eNB) 91 creates an eNB cell 10 and relays traffic between a mobile station (UE) 3 and the core network (CN) 4. A relay station (RN) 92 belongs to the base station (eNB) 91 by means of a backhaul link (BL1 in the figure) and also belongs to the core network (CN) 4 via the backhaul link (BL1). The base station (eNB) 91 and the relay station (RN) 92 can connect to a management apparatus 5 via the core network (CN) 4. The management apparatus 5 is an OAM (Operation Administration and Maintenance) system, and administers information set by the mobile telecommunications carrier. The relay station (RN) 92 is able to acquire information from the management apparatus 5. The mobile station (UE) 3 belongs to the base station (eNB) 91 or the relay station (RN) 92 by means of an access link (AL1 in the figure). The relay station (RN) 92 creates an RN cell 20 and relays traffic between the mobile station (UE) 3 and the core network (CN) 4. Details of the backhaul link and the access link are explained later.
FIG. 2 is a sequence diagram showing the outline of an RN start-up procedure described in Section 4.7.6 of 3GPP TS 36.300 V10.2.0 (2010-12), “Overall description; Stage 2 (Release 10)”. The start-up procedure includes a phase 1 and a phase 2 explained below. In the phase 1, the relay station (RN) 92 connects to a network (E-UTRAN/EPC) as a mobile station (UE). Then, the relay station (RN) 92 acquires initialization parameters from the management apparatus 5 (i.e., OAM system). The initialization parameters include a list (i.e., a donor cell list) of eNB cells to which the relay station (RN) 92 can belong as an RN by using a backhaul link (BL1). After acquiring the initialization parameters, the relay station (RN) 92 releases the network connection as the UE and thereby finishes the phase 1. Note that the base station (eNB) 91-1 to which the relay station (RN) 92 belongs in the phase 1 does not necessarily have the function of allowing a relay station (RN) to belong thereto.
In the phase 2, the relay station (RN) 92 selects one of at least one eNB cell (i.e., candidate for donor cell) indicated by the donor cell list acquired from the management apparatus 5, and belongs to a base station (eNB) 91-2 operating the selected cell as a relay station (RN). Then, the relay station (RN) 92 acquires configuration information of the backhaul link (BL1) from the base station (eNB) 91-2 to which the relay station (RN) 92 belongs, and configures the backhaul link (BL1). After finishing the procedure of the phase 2, the relay station (RN) 92 starts to operate the relay station cell (RN cell) 20.
Further, as disclosed in 3GPP TR 36.814 V9.0.0 (2010-03), “Further advancements for E-UTRA physical layer aspects”, there are three types, i.e., type 1, type 1a, and type 1b in the RN examined in 3GPP. The RN may support only one of these three types, or may change the operating mode between plural types. Alternatively, different operating modes may be used for different UEs. The type-1 RN uses the same carrier (i.e., the same frequency) for the backhaul link and the access link (in-band), and time-divides the radio resources for the backhaul link and the radio resources for the access link. The main purpose of this scheme is to avoid the interference from the access-link transmission to the backhaul-link reception in the RN.
The type-1a RN uses different carriers (i.e., different frequencies) for the backhaul link and the access link (out-band). Therefore, the type-1a RN does not require the time-division for radio resources unlike the type-1 RN, and performs mutually independent communications between the backhaul link and the access link.
Similarly to the type-1 RN, the type-1b RN uses the same frequency for the backhaul link and the access link. However, the type-1b RN does not time-divide the radio resources. This type is used on condition that the interference from the access-link transmission to the backhaul-link reception is sufficiently suppressed.
In the phase 2 of the above-described start-up procedure, the RN transmits RN identification information including an RN type to the eNB. Then, the eNB determines the control method of the backhaul link based on the RN type information included in the RN identification information. More specifically, the eNB determines whether the resources of the backhaul link should be time-divided or not based on the RN type information.
In the specification of the present application, among the eNBs, an eNB that can allow an RN to belong thereto is called “Donor eNB” (hereinafter “DeNB”). A mobile station (UE) that directly belongs to a relay station (RN) is called “RN-UE”. Further, in the discussion on 3GPP, a demand for supporting multihop RNs in the future is arising. The multihop RN is a technique that makes it possible to connect an additional relay station (RN) to a relay station (RN) that already belongs to an eNB in a cascade configuration. In this specification, in the explanation relating to the multihop technique, a relay station (RN) belonging to an eNB through a radio interface is called “upper RN” and a relay station (RN) belonging to the upper RN through a radio interface is called “lower RN” in order to distinguish them from each other. Further, in this specification, a radio interface between an eNB and an RN and between an upper RN and a lower RN is called “backhaul link”. Meanwhile, a radio interface between an eNB and an eNB-UE and between an RN and an RN-UE is called “access link”.
The inventors of the present application have made detailed examination on the donor cell selection method that is used when plural candidate donor cells are indicated by the donor cell list. In the above-described procedure disclosed in 3GPP TS 36.300 V10.2.0 (2010-12), the relay station (RN) acquires a donor cell list from a management apparatus such as an OAM system that is different from the base station (eNB). Further, the RN selects a donor cell from the donor cell list based solely on information administered by the RN (e.g., received power measured by the RN) and requests connection with the selected cell. However, the candidate donor cells indicated by the donor cell list, which is supplied from the management apparatus (i.e., OAM system) to the RN, are controlled by eNB(s) based on information administered by the eNB(s) (e.g., the load of the cells and the backhaul link configuration of other RN). Therefore, there is a possibility that a donor cell that is selected based solely on the information administered by the RN (such as received power measured by the RN) is not a desirable cell for the eNB(s) included in the mobile communication system.
For example, assume a case where an eNB generates two cells (first and second cells). When the first and second cells are both indicated by a donor cell list acquired from the management apparatus, the RN selects one of the first and second cells as a donor cell based on the reception quality of the first and second cells measured by the RN itself. In this example, assume that the first cell is selected. Then, the RN requests the eNB that the RN connects to the first cell as an RN. However, there is a possibility that the second cell is more desirable than the first cell as the cell to which the RN newly connects in consideration of the status of the first and second cells at the time when the connection is requested from the RN, i.e., in consideration of the load of the first and second cells, the connection status of other RN, and the like. However, the eNB cannot effectively control the decision on the donor cell that is made by the RN.
As a way of solving this problem, it is conceivable that the eNB (s) waits until the RN requests connection with the most suitable donor cell. Specifically, the eNB (s) may respond with the rejection to the connection request by the RN when the donor cell requested by the RN is not a suitable cell. The RN that receives the rejection response requests connection with a newly-selected donor cell. The eNB (s) repeats the rejection response as long as the donor cell requested by the RN is not a suitable cell, and waits for a connection request for the most suitable donor cell. However, in this method, the rejection response is possibly repeated for many times and therefore a long time is wasted until finding the optimal donor cell. In particular, when the donor cell is switched (backhaul link is switched) after the operation of the RN cell is started, i.e., when the donor cell is switched in a state where an RN-UE already exists, the operation of the RN is temporarily suspended. Therefore, taking a long time for the switching of the donor cell may cause a significant impact on the RN-UE.
Certain embodiments described herein provide a mobile communication system, a relay station, a base station, a control method thereof, and a program, capable of determining a donor cell based on both donor cell selection criteria on a relay station side and donor cell selection criteria on a base station side.